Our galaxy alone, has millions upon millions of stars. Within our universe, there are millions upon millions of galaxies. Mankind have known the existence of stars since they have had eyes. Although interpretations may have differed on what they were, they were always thought of as white glowing specks in the sky, but the mystery does not lie within what we can see, but what we can not see. There are an endless number of stars lighting the darkness of our universe, but the question is, what happens when one of these enormous lamps comes to the end of its life. Upon many speculations, one of the most fascinating is the black hole theory. Not any star can become a Black Hole. For example, the possibility of our sun becoming a black hole is very unlikely, simply because it is too small. Only a very large star has the possibility to become a black hole.
The definitions of black holes are sometimes controversial. In theory, a black hole is an area of highly concentrated mass. So concentrated, that no object can escape its gravitational pull. In other words, once you get caught by its gravitational pull, it is impossible to get out. The velocity required to escape from the gravitational pull is called the "escape velocity". Roughly, earth's escape velocity is about 25,000mph (11.2 kilometres/second). Earth's mass is nothing compared to the mass of a star that has the potential to become a black hole. A black hole has so much mass in such a small area, that its escape velocity is greater than the speed of light. So if were all living on earth, and earth was a black hole, we would need to go at the speed of light in order to get to the moon, which would be impossible.
Although a black hole's gravitational pull is colossal, it does have its boundary. This boundary is called the "event horizon". This event horizon is the point where the black hole's gravitational pull begins. Once you cross the event horizon, there is no way back.